The present invention relates to a device for the elimination of the zero order beam emerging from a hologram illuminated in polarized light and more particularly to a video image projector equipped with such a device.
The functions of numerous conventional optical components, lenses, filters, etc., can today be produced by means of holograms which have various advantages, particularly those of having a planar shape which does not require much space and of being inexpensive to manufacture.
However, the functioning of a hologram, as that of any optics calling for diffraction of light, is a function of the wavelength of this light. When an incident beam of white light illuminates a hologram, this dependency is at the origin of the passage of the latter by a beam called "zero order" beam which is not diffracted and therefore not deflected, in contrast to the beam diffracted by the hologram. The presence of these two beams coming out of the hologram can have the effect of contaminating the beam of diffracted light with the white light of the nondiffracted beam. This is the case in particular when the hologram produces three beams of red, green ad blue light respectively, for example, for the projection of video images in color, the zero order beam, composed of white light, affects the purity of the colors.
In order to avoid this disadvantage, it is possible to manufacture the hologram in such a way that the usable diffracted beams diverge angularly to a greater extent from the axis of the zero order beam, or to use the diffracted beams a long distance from the hologram, where these beams are sufficiently separated spatially so that the objects which they illuminate cannot also receive the light of the zero order beam.
This solution unfortunately leads to very bulky assemblies, which are obviously not suitable in apparatuses such as, for example, the video image or television projectors intended for the general public, of which the space requirement must be as small as possible.
The present invention therefore aims to produce a device for elimination of the zero order beam emerging from a hologram illuminated in white light, which is both effective and has a small space requirement.